In many situations electrically powered apparatus fulfill important and vital functions and that they operate continuously over predefined time periods is crucial. The time period which the apparatus is required to operate is generally controlled manually by an ON/OFF switch situated on or near the apparatus. However, on occasions, premature interruptions to the power supply may occur for many and various reasons including, for example, a disconnection in the power supply leads from the power source to the apparatus. Such interruptions may be extremely serious, particularly if a person's life depends on the continuous operation of the apparatus.
Accordingly, prior art systems provide a power fail detection circuit which enables an alarm each time a power fail is detected. The power fail is enabled each time the apparatus is switched off and so must be switched off at the same time.